This invention relates generally to the field of high purity silicon production and more specifically to an apparatus and process for hydrogenation of a silicon tetrahalide and silicon to trihalosilane.
Trihalosilanes such as SiHCl3 and SiHBr3 were first made by the reaction of silicon with the respective hydrogen halide, HCl or HBr, at a temperature of approximately 350° C. and at approximately atmospheric pressure. Yields from this reaction are typically very high, the literature showing yields in excess of 90%, with the remainder being silicon tetrahalide. The trihalosilane most commonly used is trichlorosilane which is purified and decomposed to form very pure silicon. This decomposition reaction also forms the silicon tetrahalide which thus builds up in the process. Various measure have been taken to minimize the production of silicon tetrahalide such as recycling the tetrahalide and using a large excess of hydrogen in the decomposition reactor. The excess tetrahalide has also been oxidized to amorphous silica with the resulting halogen gases being scrubbed. A more recent innovation, based on research by DOE/JPL and described in Breneman U.S. Pat. No. 4,676,967, converts the excess silicon tetrahalide to the trihalosilane by reacting it with hydrogen and silicon at high temperature and pressure in the presence of a copper catalyst.
The prior hydrogenation technology disclosed in Breneman U.S. Pat. No. 4,676,967 conducts the reaction between silicon, silicon tetrachloride and hydrogen at about 400° C.-600° C. and between 300 and 600 psi in a fluidized bed reactor containing a distribution means above a mixing plenum. In commercial embodiments of the above patent the inlet gas streams of silicon tetrachloride and hydrogen are preferentially heated separately to different temperatures and pressures then mixed in the mixing plenum before passing through the distribution means to form bubbles which contact and fluidize the finely divided silicon. The hydrogen is heated to 500° C. at 325 psig while the silicon tetrachloride is heated to 500° C. at 575 psig, which is above the 530 psig critical pressure of silicon tetrachloride, and then depressurized to 325 psig on mixing with the hydrogen.
The commercial reactors designed according to Breneman have a lesser yield than smaller scale laboratory tests, such as the ones by Ingle, discussed in “Kinetics of the Hydrogenation of Silicon Tetrachloride, W. Ingle and M. Peffley, J. Electrochem Soc may 1985, pg 1236-1240” and the yield decreases with time with the ultimate need to dump the entire bed of silicon and start over with fresh material. Delaying the mixing to the mixing plenum increases the capital cost and operating cost because it makes heat recovery from the hot effluent gases more difficult and causes operational problems. The silicon tetrahalide frequently contains significant traces of the trihalosilane and dihalosilane which can decompose to silicon inside the tetrahalide heaters causing plugging. Using two separate preheaters increases the capital cost compared to a single train and makes it more difficult to recover heat from the exhaust gas which increases the use of utilities. The provision of a distribution means increases capital cost.
The lower yield of commercial reactors can be attributed to the design of the distribution means and selection of typical fluidized bed operating conditions which cause bypassing of the silicon by large bubbles of reacting gas which means some of the reacting gas has a very short residence time. From Ingle it can be seen that yield depends on residence time but eventually reaches a plateau; thus a mixture of gas with low and high residence time will have a lower yield than one with the same average residence time where all the gas has the same residence time. The decrease in yield with time is caused by buildup of impurities on the surface of the silicon bed materials, as discussed in Bade, S., “Mechano-chemical reaction of metallurigal grade silicon with gaseous hydrogenchloride in a vibration mill, S. Bade et al, Int. J. Mineral Processing 44-45 (1996) 167-179”. The design of the plenum according to standard fluidized bed practice minimizes the amount of attrition which is thus inadequate to provide sufficient new reactive surface. In order to improve the yield it is important to define the process conditions in a more specific way than the very general definition of a temperature between 400° C.-600° C. and 300 to 600 psi. The key criteria is the ratio of the superficial velocity, U, to the minimum bubbling velocity, Umb., (U/Umb). The superficial velocity is the actual volumetric flow, adjusted for operating temperature and pressure, divided by the cross-sectional area of the vessel. The minimum bubbling velocity, Umb. is the minimum superficial velocity at which bubbles are formed, and is of importance because bubbles move faster than gas in the voids between particles and thus have less residence time. This velocity depends on the gas properties and on the size, shape and density of the granular particles. There are many ways to calculate this property, including computer programs. For large, greater than 200 micron, particles the Umb is essentially the same as the superficial velocity at minimum fluidization, Umf. Providing more comminution is known to be valuable in the similar process of reacting metallurgical grade silicon with hydrogen chloride as is discussed in Bade. The comminution increases reaction rate and yield but the Bade method of using a ball mill makes it difficult and expensive to implement at the high pressure, 500 psig, and high temperature, 500° C., needed for this reaction.
Thus what is desired is a reactor and a set of operating conditions which provide more even gas-solids contacting, more comminution, better energy recovery and lower capital and operating cost.